This application is 371 or PCT/GB 96/02308 filed Sep. 16, 1996.
This invention relates to a position determining system using radio or other broadcast transmissions. More particularly, it relates to a system using the Global System for Mobile Communications (GSM) or other digital transmission systems.
In our European Patent EP-B-0 303 371 we describe a radio navigation and tracking system, now known as xe2x80x98CURSORxe2x80x99, which uses the spatial coherence of the signals from several radio transmitters to determine the position of a roving receiver. Its principles are explained in the above patent specification where it is shown how the signals received directly by the roving receiver (rover) are compared with those received by a fixed base station whose location is known (base) to determine their phase difference, and hence the difference in range of the base and rover from each transmitter. Three such measurements made on independent transmitters are needed for navigation and tracking in two dimensions to fix the position of the rover relative to the base station and network of transmitters. The unknown quantities calculated for each new position are the spatial x and y co-ordinates of the rover together with the phase offset between the local oscillators in the equipment of the two receivers. A further patent specification, WO 94/28432, shows how the same principles may be applied in tunnels and other shielded spaces such as underground car parks. Duffett-Smith and Woan (Journal of Navigation, 45, 157, 1992) describe a particular implementation of the phase-measuring system in which the signals from three or more medium-wave AM public broadcast stations are used to track the position of a vehicle in and around Cambridge, UK, at speeds up to 110 km hxe2x88x921, with a precision of about 5 m. Once of the advantages of such a system lies in the fact that no additional costly infrastructure of coherent radio transmitters needs to be set up for CURSOR operation. On the contrary, CURSOR is able to use the signals from any independent radio transmitters set up for any purpose.
EP-B-0 303 371 also explains how the wider bandwidth signals of modulated transmissions may be used to measure the time difference between the signals received from each transmitter at the base station and the rover. In this case, the position of the peak in the cross correlation can be used as an estimator of the time difference between the two received signals, and hence the difference in distance from the transmitter of the base and rover. As with the phase-measuring system, three such measurements made on three widely-spaced transmitters suffice to calculate the spatial x and y co-ordinates of the rover together with the time offset between the oscillators in the two receiving stations.
Mobile telephones are increasingly using GSM and other digital techniques, and it would be an advantage to add time-measuring CURSOR positioning technology to provide additional services for users. However, signals radiated by digital telephone transmitters are complex and therefore there are problems to be overcome in attempting to combine the technologies.
According to the present invention there is provided a position determining system, for receiving broad band signals transmitted by a number of transmission sources equal at least to the number of dimensions in which the movement of a roving object is to be monitored, the system comprising
a pair of receiving stations, in use the first of the receiving stations being at a known position and the second being located on the roving object;
a position determining processor;
means for passing a link signal from each of the receiving stations to the position determining processor, the link signal containing information about the signals received at the receiving station from the transmission sources;
wherein each of the receiving stations is arranged to receive the signals from the respective transmission sources substantially simultaneously, and the position determining processor is arranged to compare the information received from the one receiving station about the signals received at the one receiving station from the transmission sources with the information received from the other receiving station about the signals received at the other receiving station from the transmission sources, and to determine the time delay between the respective signals received at both receiving stations in order to determine the position of the roving object.
The receiving stations may receive the signals from the respective transmission sources sequentially and in the same sequence as each other.
The invention also includes a method of determining the position of a roving object, the method comprising transmitting a broadband signal from a number of transmission sources equal at least to the number of dimensions in which the movement of a roving object is to be monitored; receiving the signals at a pair of receiving stations, in use a first of the receiving stations being at a known position and a second being located on the roving object, each of the receiving stations being arranged to receive the signals from the respective transmission sources substantially simultaneously; passing link signals from each of the receiving stations to a position determining processor, the link signals containing information about the signals received at the respective receiving station from the transmission sources; comparing the information received at the position determining processor from the one receiving station about the signals received at the one receiving station from the transmission sources with the information received from the other receiving station about the signals received at the other receiving station from the transmission sources; and determining the time delay between the respective signals received at both receiving stations in order to determine the position of the roving object.
Additionally, the invention includes a method of estimating the offset in time of receipt of a broadcast signal received at two locations, where the signal received at the one location may have been subject to corruption by multipath effects, the method comprising auto-correlating the signals received at the one location; auto-correlating the signals received at the other location; cross-correlating the signals received at the one and other locations; constructing a template comprising that portion of the auto-correlation of the signals received at the other location corresponding to the negative time axis, and that portion of the auto-correlation of the signals received at the one location corresponding to the positive time axis; and measuring the offset at which the template best fits the measured cross-correlation of the signals received at the one and other locations as an estimate of the time offset between the signals received at the two locations.
In some systems, each of the receiving stations is also arranged to receive a second signal from one or more of the transmitters, said second signal being used to enable compensation for changes in equipment offsets during receipt of the sequentially received signals.
The position determining processor may be co-located with one of the receiving stations or may be located remotely.
In one embodiment, the link signal from one of the receiving stations is passed to the other receiving station and from the other receiving station to the position determining processor.
Preferably, for locating purposes, a signal providing information about the position of the roving object is passed from the position determining processor to at least one of the receiving stations.
The system may further comprise one or more monitoring stations and a signal providing information about the position of the roving object may then be passed from the position determining processor to one or more of the monitoring stations.
To provide enhanced functionality, a database server may be connected to the position determining processor, the database server containing data elements relating to a plurality of known positions; and the system further comprises means for passing information about the position of the roving object determined by the position determining processor to the database server; means for retrieving data elements related to the position determined by the position determining processor; and means for passing the data elements to one of the receiving stations or to one or more of the monitoring stations.
The or each receiving station or the or each monitoring station preferably includes a display and the position of the roving object is displayed on the display which may be a dot matrix display.
The database server may contain graphical information and the graphical information is passed to a receiving station or monitoring station and displayed on the display to indicate the position of the roving object.
The transmission sources and receiving stations preferably comprise components of a digital cellular telephone network such as a GSM network. It is advantageous if the receiving stations monitor the strength of the signals of plural transmission sources and select a plurality of sufficiently strong signals for reception.
The quasi-synchronisation between the signals received from the respective transmission sources is preferably achieved by monitoring receipt of a specific portion of the transmitted signals, but may be achieved, alternatively, by means independent of the transmitted signals such as a local clock signal.
The system preferably includes a series of regionally, nationally, or even internationally networked fixed receiving stations to provide wide coverage and usage of the positioning system.
The receiving stations may be capable of receiving two or more channels simultaneously and it may be advantageous to repeat the reception of signals from plural transmission sources.
The description below sets out how the principles of the time-measuring CURSOR system may be applied to a digital radio network, such as the GSM telephone system, to enable the position of a receiver, such as a portable telephone handset, to be measured relative to the transmitter network. The equipment used in such a network already incorporates most of that required for position fixing operation, so that its implementation may be achieved with little extra cost. The precision of each position measurement is proportional (amongst other factors) to the inverse of the bandwidth of the signals; for a GSM single channel of 200 KHz bandwidth, the precision is about 50 m. Some improvement on this figure may be obtained in cases where more than three GSM transmitters can be received at both base and rover, they may all then be used in the position determining process.